Corrosion-resistant alloys

ABSTRACT

A workable, machinable alloy having good corrosion resistance to sulfuric acid solutions over a wide range of acid concentrations. The alloy consists essentially of between about 35.2 and about 46.2 percent by weight nickel, between about 14.9 and about 25.3 percent by weight chromium, between about 6.7 and about 14.5 percent by weight molybdenum, between about 4.3 and about 8.1 percent by weight copper, between about 14.06 and about 30.62 percent by weight iron, up to about 1.50 by weight manganese, up to about 0.11 percent by weight carbon, up to about 1.00 percent by weight silicon, up to about 0.010 percent by weight boron, up to about 1.00 percent by weight niobium, up to about 2.00 percent by weight tantalum, and up to about 1.00 percent by weight titanium. It is also essential that the sum of the molybdenum content and copper content be at least about 11.7 percent by weight. The sum of the niobium plus one-half the tantalum content should not exceed about 1.00 percent by weight.

United States Patent [191 Culling 51 Oct. 29, 1974 [73] Assignee: Carondelet Foundry Company, St.

Louis, Mo.

[75] Inventor:

[22] Filed: Sept. 24, 1973 [21] App]. No.: 399,687

[52] US. Cl. 75/134 F, 75/122, 75/134 C [51] Int. Cl. C22c 31/00 [58] Field of Search 75/134 F, 134 C, 122, I71

[56] References Cited 7 UNITED STATES PATENTS 2,860,968 11/1958 Boegehold et al. 75/122 2,938,786 5/1960 Johnson 75/171 3,758,294 9/1973 Bellot et al. 75/122 3,758,296 9/1973 Johnson 75/122 3,759,704 9/1973 Culling 75/122 FOREIGN PATENTS OR APPLICATIONS 36,659 11/1970 Japan 75/134 F Primary ExaminerL. Dewayne Rutledge Assistant ExaminerE. L. Weise Attorney, Agent, or Firml(oenig, Senniger, Powers and Leavitt [57] ABSTRACT A workable, machinable alloy having good corrosion resistance to sulfuric acid solutions over a wide range of acid concentrations. The alloy consists essentially of between about 35.2 and about 46.2 percent by weight nickel, between about 14.9 and about 25.3 percent by weight chromium, between about 6.7 and about 14.5 percent by weight molybdenum, between about 4.3 and about 8.1 percent by weight copper, between about 14.06 and about 30.62 percent by weight iron, up to about 1.50 by weight manganese, up to about 0.1 1 percent by weight carbon, up to about 1.00 percent by weight silicon, up to about 0.010 percent by weight boron, up to about 1.00 percent by weight niobium, up to about 2.00 percent by weight tantalum, and up to about 1.00 percent by weight titanium. It is also essential that the sum of the molybdenum content and copper content be at least about 11.7 percent by weight. The sum of the niobium plus one-half the tantalum content should not exceed about 1.00 percent by weight.

4 Claims, No Drawings M BACKGROUND OF THE INVENTION This invention relates to the field of corrosionresistant alloys and particularly to novel alloys suitable for use with a wide range of sulfuric acid concentrations.

Not only are sulfuric acid solutions very corrosive generally, but the nature of their corrosive properties varies markedly with both acid concentration and temperature. In most concentration ranges, sulfuric acid solutions are strong reducing acids. In higher concentrations, however, and particularly at elevated temperatures, sulfuric acid solutions can be oxidizing and in some instances highly oxidizing.

As a consequence of this variability in the corrosive properties of sulfuric acid, few materials are available which are reasonably resistant to sulfuric acid solutions over a wide range of concentrations. A relatively large number of available materials exhibit reasonable resistance to either dilute sulfuric acid solutions having an acid strength of less than about 20 percent by weight or to concentrated solutions having acid strengths of greater than 80 percent by weight. A lesser number of materials are effective for the intermediate and generally more corrosive concentration range of 20 to 80 percent; and even fewer metal alloys are commercially useful in contact with sulfuric acid solutions ranging from strengths below 20 to greater than 80 percent, particularly when exposed to elevated temperatures.

Of the known alloys which are demonstratedly effective over wide ranges of sulfuric acid concentrations, many contain relatively high proportions of nickel and are thus rather expensive. Many of such alloys are relatively brittle and cannot be readily machined or wrought, and are thus suitable only for use in castings which need not be machined.

SUMMARY OF THE INVENTION Among the several objects of the present invention, therefore, may be noted the provision of alloys which are resistant to sulfuric acid solutions over a wide range of acid concentrations; the provision of such alloys which retain good corrosion-resistant properties at elevated temperatures; the provision of such alloys which are resistant under conditions which may become highly oxidizing; the provision of alloys which may be readily machined and wrought; and the provision of alloys which may be formulated at moderate cost. Other objects and features will be in part apparent and in part pointed out hereinafter.

The present invention is therefore directed to a corrosion-resistant alloy consisting essentially of between about 35.2 and about 46.2 percent by weight nickel, between about 14.9 and about 25.3 percent by weight chromium, between about 6.7 and about 14.5 percent by weight molybdenum, between about 4.3 and about 8.1 percent by weight copper, between about 14.06 and about 30.62 percent by weight iron, up to about 0.1 l percent by weight carbon, up to about 1.00 percent by weight silicon, up to about 1.50 percent by weight manganese, up to 0.010 percent by weight boron, up to about 1.00 percent by weight niobium, up to about 2.00 percent by weight tantalum, and up to about 1.00 percent by weight titanium. The sum of the molybdenum content and the copper content is at least about 11.7 percent by weight, while the sum of the niobium content and one-half the tantalum content is no greater than about 1.00 percent by weight.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The alloys of the present invention exhibit highly satisfactory corrosion resistance in sulfuric acid solutions ranging from 10 by weight to 93 percent by weight and higher. The high resistance of these alloys is maintained at elevated temperatures, for example 176F. and under conditions which are highly oxidizing. Good corrosion resistance is also exhibited by these alloys under the strongly depassivating effect of chloride ions, for example, in hydrochloric acid solutions at room temperature. The alloys of the invention, moreover, have advantageous mechanical properties and can be rolled, drawn, machined or otherwise worked in accordance with conventional forming and fabricating techniques. While the alloys possess outstanding corrosion resistance properties, their formulation cost is moderate. The proportions of iron which can be tolerated in these alloys are high enough to allow the use of ferro alloys for their formulation. The essential components of the alloys of the invention are:

Per cent By Weight Nickel 35.2 to 46.2 Chromium 14.9 to 25.3 Molybde- 6.7 to 14.5 num Copper 4.3 to 81 Iron 14.06 to 30.62

A critical parameter of the alloys of the invention is the sum of the molybdenum and copper content, which must be at least about 1 1.7 percent by weight. A synergistic effect of copper and molybdenum on the corrosion resistance of the alloys is realized when the total concentration of copper plus molybdenum exceeds 1 1.7 percent by weight.

Various other components are optionally present in the alloys of the invention, and the presence of certain of these components is advantageous especially for certain particular applications.

Per cent By Weight Carbon Up to 0.1 1

Silicon Up to 1.00 Manganese Up to 1.50

Boron Up to 0.010 Niobium Up to 1.00 Tantalum Up to about 2.00 Titanium Up to about 1.00

The sum of the niobium and one-half of the tantalum content generally should not exceed about 1.00 percent by weight.

The presence of a small proportion of boron, for example, about 0.005 percent by weight is desirable where workability of the alloy is particularly important. The presence of up to 0.010 percent by weight boron enhances elongation values and hence workability. The presence of up to 1.50 percent by weight manganese also contributes to the workability of the alloy.

Up to 1.00 percent by weight titanium, or niobium plus one-half the tantalum, may be included to eliminate the need for solution heat treatment of the alloys following welding, casting or cold working. Titanium, niobium and tantalum are effective for stabilizing carbide which may otherwise cause intergranular corrosion in alloys which are not heat treated subsequent to welding. Where carbide stabilization is important, the alloys preferably contain at least 10 parts by weight niobium plus tantalum or at least five parts by weight titanium per one part by weight carbon. Titanium, niobium and tantalum also have a beneficial effect upon fabricability.

tent, and a corresponding lower allowable iron content than the alloys of this invention. Moreover, most lllium alloys are rather brittle and hard and are not as susceptible to machining and cold working as are the alloys of the present invention.

The following examples illustrate the invention.

EXAMPLE 1 100-1b. heats of seven different alloys were prepared in accordance with the invention. Each of these heats was dead-melted in a 100-lb. high frequency induction furnace. The composition of these alloys is set forth in Table 1.

Table I Percent by Weight of Alloving Elements Alloy lron. By

No. Ni Cr Mo Cu C Si Mn B Ti Difference High ductility and superior fabricability properties are achieved if the alloy composition falls within the following preferred range:

Nickel 35.70% to 46.17% Chromium 18.19% to 25.287: Molybdenum 6.74% to 9.66% Copper 4.37% to 7.017: Carbon 0.11% maximum Molybdenum Copper 11.71% minimum to 16.67% maximum Silicon 1.00% maximum Manganese Up to 1.50% maximum Niobium +Tantalum 10 X Carbon to 1.00% maximum Titanium 5 X Carbon to 1.0071 maximum Iron 14.067: to 30.627:

It will be noted that these preferred alloys are also especially economical to formulate because of the relatively low proportions of molybdenum which they contain. Molybdenum is the most expensive of the elements of which these alloys are constituted. The relatively high proportions of chromium contained by these preferred alloys also promotes good corrosion resistance to a wide variety of corrosive media.

The corrosion resistance of the alloys of the invention compares favorably with that of other commercially available alloys designed for use in sulfuric acid, including, for example, alloys such as those sold under the lllium trade designation. The lllium alloys, which are adapted for use over a wide range of sulfuric acid concentrations, have a higher critical element con- Standard physical test blocks and corrosion test bars were prepared from each heat. The mechanical properties of each of these alloys was then measured in the as- The corrosion test bars were annealed for 30 minutes at 1,950F. and oil-quenched prior to machining into 1 /2 inch diameter by 4 inch high discs having a /s inch diameter hole in the center. Care was exercised during machining to obtain an extremely smooth surface on the discs. Twelve to fourteen discs were obtained for each alloy.

These discs were used in the comparative corrosion tests described hereinafter, comparing the performance of the alloys of the invention with a number of commercially available alloys. The compositions of the commercially available. alloys which were used in these tests and the respective trade designations under which they are marketed are set forth in Table 111.

Table 111 Commercial Alloys Utilized in Comparative Corrosion Tests lron. By Alloy Ni Cr Mo Cu C Si Mn B Ti Difference Carpenter 20 29 20 2.3 3.3 .05 .50 .95 44. No.20Cb3 33 20 2.3 3.3 .05 .50 .95 005 1.0 39.6 PHA 9 20 4.0 .04 3.5 .5 63.

Table Ill- Continued w Commercial Alloys Utilized in Comparative Corrosion Tests Iron, By Alloy N1 Cr Mo Cu C Si Mn B Ti Difference PHSSB 9 20 5.0 3.5 .04 1.5 .5 61. PHSSC 9 20 4.0 3.0 .04 3.5 .5 60. 3 16 Stainless I2 20 2.5 .05 .8 .7 64. lllium o 56 22.5 6.4 6.5 .20 .65 1.25 6.5 lllium 9s 55 28 8.5 5.5 .05 .90 1.25 1. lllium B 50 211 8.5 5.5 .05 5.5 1.25 .5 2.5 CD4MCU 5 25 2 3 .05 .5 .5 64. Worthite 24 20 3.0 1.75 .07 3.50 1.00 46.7

EXAMPLE 2 Table lV-Continued Comparative corrosion tests were run in percent by at Disc samples of Carpenter 2O Carpenter Cb3 20 3 Losses in Inches of Penet t Cooper PHSSA, Cooper PHSSB, Cooper PHSSC, Alloy N6. Per Year 01w.) CD4MCu, Worthite, 316 Stainless Steel, lllium G, ll- 3 Q0030 lium 98, and 304 Stainless Steel were prepared in the 4 0.0027 manner described in Example 1. Residual machining 2 8-8823 oil and dirt were removed from all of the sample discs 25 7 0.0016 by cleaning them with a small amount of carbon tetra- Carpenter 20 0005 N0. 20Cb3 0.0045 chloride. The discs were then rinsed in water and dried. PHSSA 0023 PHSSB 0.0006 Each disc was weighed to the nearest 10,000th of a g gi g 888% gram and then suspended in a beaker by a piece of thin Worthite 0.030 plastic wire hooked through the center hole of the disc g f a' 8- 32 and attached to a glass rod which rested on top of the "Hum 98 1 beaker. Sufficient 10 percent sulfuric acid solution was 304 Stainless then added to the beaker so that the entire sample was immersed. The temperature of the acid was thermostat- 35 ically controlled at 176F. by means of a water bath, EXAMPLE 3 and each beaker was covered with a watch glass to mmimize evaporation. Comparative tests were conducted 1n 25 percent sul- Af precisely 6 hours, the sample discs were furic acid solution at l76F. Sample d1scs were premoved from the sulfuric acid solution and cleaned of pared and tested In the nner descrlbed in Example corrosion products. Most samples were cleaned suffif that a 25 Percent Sulfunc 9 w ciently with a small nylon bristle brush and tap water. used Place of 10 Percent sulflll'lc acid 501mm Those samples on which the corrosion product was too The results of test are Set forth m Table heavy for removal with a nylon brush were cleaned T bl V with a l-to-l solution of hydrochloric acid and water. After the corrosion products had been removed, each CORROSION RATES IN 25% H2504 AT 76F (80C) disc was again weighed to the nearest 10,000th of a I gram. The corrosion rate of each disc, in inches per 1mm Inches Peneimm Alloy No. Per Year (l.P.Y.) year, was calculated by the followmg formula in accordance with ASTM specification 01-67. g 1 3 010005 R.,,,, 0.3937 (W W;)/ATD 4 0.0 16 where 2 (1000 R corrosion rate in inches per year 7 8:8888 W original weight of sample C arpenter 20 0.010 W,= final weight of sample gfi' gg 88;; A area of sample in square centimeters P145513 0:050 T duration of test in years 351% 8 35 U D density of alloy in g./cc. worthite 0020 Results of this corrosion test are set forth in Table IV. 316 Stainless 0.200 lllium 6 0.004 lllium 98 0.007 Table IV CORROSION RATES IN 10% 11,50. AT l76F. (80C.) EXAMPLE 4 Losses in Inches of Penetration Alloy No. Per Year (I.P.Y.)

Comparative tests were conducted in 40 percent sulfuric acid solution at 176F. Sample discs were prepared and tested in the manner described in Example 2, except that a 40 percent sulfuric acid solution was used in place of the 10 percent sulfuric acid solution. The results of this test are set forth in Table VI.

Table VI CORROSION RATES IN 40% H 80 AT 176F. (80C.)

Losses in Inches of Penetration Alloy No. Per Year (I.P.Y.)

I 0.002l 2 0.0027 3 0.0022 4 0.0062 5 0.001 I 7 0.0000 Carpenter 20 0.016

No.20Cb3 0.009 PHSSA l PI -155B PHSSC Worthite (X019 lllium G 00062 lllium 98 0-0054 EXAMPLE Comparative tests were conducted in 50 percent sulfuric acid solution at 176F. Sample discs were prepared and tested in the manner described in Example 2, except that a 50 percent sulfuric acid solution was used in place of the percent sulfuric acid solution.

The results of this test are set forth in Table VII.

Table VII CORROSION RATES IN 50% H 80 AT 176F. (80C.)

Losses in Inches of Penetration Alloy N0. Per Year (I.P.Y.)

1 0.0000 2 0.0000 3 0.0027 4 0.0019 5 0.0016 6 0.0000 7 0.0103 Carpenter 20 0.017 No. 20Cb3 0.008 PHSSB 0.l PHSSC 0.l Worthite 0.029 lllium G 0.005 lllium 98 0.005

EXAMPLE 6 Comparative tests were conducted in 60 percent sulfuric acid solution at 176F. Sample discs were prepared and tested in the manner described in Example 2, except that a 60 percent sulfuric acid solution was used in place of the 10 percent sulfuric acid solution. The results of this test are set forth in Table VIII.

Table VIII CORROSION RATES IN 60% H 50 AT l76F. (80C.)

Losses in Inches of Penetration Alloy No. Per Year (l.P.Y.)

Carpenter 20 0.018

No. 20Cb3 0.010

Table VIII-Continued V d CORROSION RATES IN 60% H250. AT 176F. (80C.)

Losses in Inches of Penetration Alloy No. Per Year (I.P.Y.)

' PHSSA 0.20

PHSSB 020 Worthite lllium G 0-0039 lllium 98 0.0048

lllium 8 0-032 EXAMPLE 7 Comparative tests were conducted in percent sulfuric acid solution at 176F. Sample discs were prepared and tested in the manner described in Example 2, except that a 75 percent sulfuric acid solution was used in place of the 10 percent sulfuric acid solution. The results of this test are set forth in Table IX.

Table IX CORROSION RATES IN 7.5% H50, AT 176F. (C.)

Losses in Inches of Penetration Alloy No. Per Year (I.P.Y.)

1 0.0062 2 0.0700 3 0.0068 4 0.0443 5 0.0030 6 0.0000 7 0.0036 Carpenter 20 0.050 No. 20Cb3 0.050 PHSSA 0.15 PH55B 0.12 PH55C 0.l0 316 Stainless 1.2 lllium G 0.028 lllium 98 0.01 1 lllium B 0.026

EXAMPLE 8 Comparative tests were conducted in 93 percent sulfuric acid solution at 176F. Sample discs were prepared and tested in the manner described in Example 2, except that a 93 percent sulfuric acid solution was used in place of the 10 percent sulfuric acid solution. The results of this test are set forth in Table X.

Table X CORROSION RATES IN 93% H 50 AT 176F. (80C.)

' Losses in Inches of Penetration Alloy No.

EXAMPLE 9 Table XI CORROSION RATES IN 40% H 80 PLUS HNO AT 176F. (80C.)

Losses in Inches of Penetration Alloy No. Per Year (I.P.Y.)

As these results indicate, the alloys of the invention provide very satisfactory corrosion resistance even in environments containing relatively strong oxidizing agents.

EXAMPLE l0 Table XII CORROSION RATES IN 10% HCI AT ROOM TEMPERATURE Losses in Inches of Penetration Alloy No. Per Year (I.P.Y.)

0.0122 0.0092 0.0124 0.01 1 l 0.0149 0.0089 0.01 l I EXAMPLE 1 l Comparative corrosion tests were conducted in 20 percent hydrochloric acid at room temperature. Corrosion sample discs were prepared and tested in the manner described in Example 2, except that the test solution was 20 percent hydrochloric acid at room temperature. The results of these tests are set forth in Table X111.

10 Table X111 CORROSION RATES IN 20'? HCI AT ROOM 'I'ISMPERATURl-I Losses in Inches of Penetration Alloy No. Per Year (l.P.Y.)

The results of the tests of Examples 10 and l 1 illustrate that the alloys of the invention are corrosion-resistant even under the highly depassivating influence of solutions such as hydrochloric acid. i

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above products without departing from the scope of the invention,

it is intended that all matter contained in the above de-' scription shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A corrosion-resistant alloy consisting essentially of between about 35.2 and about 46.2 percent by weight nickel, between about 14.9 and about 25.3 percent by weight chromium, between about 6.7 and about 14.5 percent by weight molybdenum, between about 4.3 and about 8.1 percent by weight copper, between about 14.06 and about 30.62 percent by weight iron, up to about 0.1 1 percent by weight carbon, up to about 1.00 percent by weight silicon, up to about 1.50 percent by weight manganese, up to about 0.010 percent by weight boron, up to about 1.00 percent by weight niobium, up to about 2.00 percent by weight tantalum, and up to about 1.00 percent weight titanium, the sum of the molybdenum content and the copper content being at least about 11.7 percent by weight and the sum of the niobium content and one-half of the tantalum content being no greater than about 1.00 percent by weight.

2. An alloy as set forth in claim 1 wherein the sum of the niobium content and the tantalum content is at least about 10 times the carbon content.

3. An alloy as set forth in claim 1 wherein the titanium content is at least about five times the carbon content.

4. An alloy as set forth in claim 1 wherein the nickel content is between about 35.70 and about 46.17 percent by weight, the chromium content is between about 18.19 and about 25.28 percent by weight, the molybdenum content is between about 6.74 and about 9.66 percent by weight, the copper content is between about 4.37 and about 7.01 percent by .weight, the titanium content is between about five times the carbon content and about 1.00 percent by weight, the sum of the tantalum and niobium content is between about ten times the carbon content and about 1.00 percent by weight, and the sum of the molybdenum and copper content is between about 11.7 percent and about 16.67 percent by weight. 

1. A CORRESION-RESISTANT ALLOY CONSISTING ESSENTIALLY OF BETWEEN ABOUT 35.2 AND ABOUT 46.2 PERCENT BY WEIGHT NICKEL, BETWEEN ABOUT 14.9 AND ABOUT 25.3 PERCENT BY WEIGHT CHROMIUM, BETWEEN 6.7 AND ABOUT 14.5 PERCENT BY WEIGHT MOLYBDENUM, ETWEEN ABOUT 4.3 AND ABOUT 8.1 PERCENT BY WEIGHT COPPER, BETWEEN ABOUT 14.06 AND ABOUT 30.62 PERCENT BY WEIGHT IRON, UP TO ABOUT 0.11 PERCENT BY WEIGHT CARBOB, UP TO ABOUT 1.00 PERCENT BY WEIGHT SILICON, UP TO ABOUT 1.50 PERCENT BY WEIGHT MANGANESE, UP TO ABOUT 0.010 PERCENT BY WEIGHT BORON, UP !O ABOUT 1.00 PERCENT BY WEIGHT NIOBIUM, UP TO ABOUT 2.00 PERCENT BY WEIGHT TANTALYM, AND UP TO ABOUT 1.00 PERCENT WEIGHT TITANIUM, THE SUM OF THE MOLYBDENUM, CONTENT AND THE COPPER CONTENT BEING AT LEAST ABOUT 11.7 PERCENT BY WEIGHT AND THE SUM OF THE NIOBIUM CONTENT AND ONE-HALF OF THE TANTALUM CONTENT BEING NO GREATER THAN ABOUT 1.00 PERCENT BY WEIGHT.
 2. An alloy as set forth in claim 1 wherein the sum of the niobium content and the tantalum content is at least about 10 times the carbon content.
 3. An alloy as set forth in claim 1 wherein the titanium content is at least about five times the carbon content.
 4. An alloy as set forth in claim 1 wherein the nickel content is between about 35.70 and about 46.17 percent by weight, the chromium content is between about 18.19 and about 25.28 percent by weight, the molybdenum content is between about 6.74 and about 9.66 percent by weight, the copper content is between about 4.37 and about 7.01 percent by weight, the titanium content is between about five times the carbon content and about 1.00 percent by weight, the sum of the tantalum and niobium content is between about ten times the carbon content and about 1.00 percent by weight, and the sum of the molybdenum and copper content is between about 11.7 percent and about 16.67 percent by weight. 